Constant-bandwidth linear variable gain amplifier

ABSTRACT

The present invention is directed electrical circuits. According to a specific embodiment, the present invention provides a variable gain amplifier that includes a first switch, which includes drain terminal coupled to an inductor. A second switch is configured in parallel to the inductor, and the resistance value of the second switch is adjustable in response to a control signal. There are other embodiments as well.

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BACKGROUND OF THE INVENTION

The present invention is directed to electrical circuits.

A variable gain amplifier (VGA) has many applications. Typically,variable-gain or voltage-controlled amplifier is an electronic amplifierthat varies its gain depending on a control voltage (CV). VGAs have manyapplications, including data communication, audio level compression,synthesizers, amplitude modulation, and others. For example, a VGA canbe implemented by first creating a voltage-controlled resistor (VCR),which is used to set the amplifier gain. The VCR can be produced by oneor more transistors with simple biasing. In certain implementation, VGAsare implemented using operational transconductance amplifiers.Sometimes, VGAs are implemented for automatic gain control (AGC)applications. Typically, VGA performance can be measured in terms ofgain range, linearity of electrical characteristics, distortion,tunabiltiy, and bandwidth.

Over the past, many types of conventional variable gain amplifiers havebeen proposed and implemented in different applications. Unfortunately,existing variable gain amplifiers are inadequate, as explained below. Itis thus desirable to have new and improved variable gain amplifiers.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed electrical circuits. According to aspecific embodiment, the present invention provides a variable gainamplifier that includes a first switch, which includes drain terminalcoupled to an inductor. A second switch is configured in parallel to theinductor, and the resistance value of the second switch is adjustable inresponse to a control signal. There are other embodiments as well.

According to an embodiment, the present invention provides a variablegain amplifier device, which includes a first switch comprising a firstgate terminal and a first drain terminal. The first gate terminal iscoupled to a first input voltage. The device also includes a secondswitch comprising a second gate terminal and a second drain terminal.The second gate terminal is coupled to a second input voltage. Thedevice also includes a first output terminal coupled to the first drainterminal. The device additionally includes a second output terminalcoupled to the second drain terminal. The device further includes afirst inductor coupled to the first drain terminal. The deviceadditionally includes a second inductor coupled to the second drainterminal. The device further includes a third switch comprising a thirdgate terminal and configured in parallel to the first inductor. Thedevice additionally includes a fourth switch comprising a fourth gateterminal and configured in parallel to the second inductor.

According to another embodiment, the present invention provides avariable gain amplifier device, which includes a first switch comprisinga first base terminal and a first collector terminal. The first baseterminal is coupled to a first input voltage. The device furtherincludes a second switch comprising a second base terminal and a secondcollector terminal. The second base terminal is coupled to a secondinput voltage. The device additionally includes a first output terminalcoupled to the first collector terminal. The device further includes asecond output terminal coupled to the second collector terminal. Thedevice also includes a first inductor coupled to the first collectorterminal. The device additionally includes a second inductor coupled tothe second collector terminal. The device also includes a third switchcomprising a first gate terminal and configured in parallel to the firstinductor. The device also includes a fourth switch comprising a secondgate terminal and configured in parallel to the second inductor.

According to yet another embodiment, the present invention provides areceiver apparatus, which includes an input terminal for receivingincoming data signals. The receiver apparatus also includes a variablegain amplifier coupled to the input terminal. The variable gainamplifier includes a first switch and an inductor. The input switch iscoupled to the input terminal. The input switch includes a sourceterminal coupled to a variable resistor and a drain terminal coupled tothe inductor. The variable resistor is coupled to a first controlsignal. The variable gain amplifier further includes a second switchconfigured in parallel to the inductor. The second switch is coupled tosecond control signal. The apparatus also includes an analog to digitalconverter (ADC) configured to generate digital signal based on theincoming data signal, the ADC being coupled to the drain terminal.

It is to be appreciated that embodiments of the present inventionprovide many advantages over conventional techniques. Among otherthings, variable gain amplifiers according to the present invention caneffectively improve device linearity and bandwidth, while providinglarge gain control range. When incorporated into various types ofcommunication systems, amplifier according to the present invention caneffectively improve overall system performance.

Embodiments of the present invention can be implemented in conjunctionwith existing systems and processes. For example, variable gainamplifiers and communication systems according to the present inventioncan be manufactured using existing manufacturing equipment andtechniques. Additionally, embodiments of the present invention can bereadily adopted into existing communication and other applications.

The present invention achieves these benefits and others in the contextof known technology. However, a further understanding of the nature andadvantages of the present invention may be realized by reference to thelatter portions of the specification and attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following diagrams are merely examples, which should not undulylimit the scope of the claims herein. One of ordinary skill in the artwould recognize many other variations, modifications, and alternatives.It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this process andscope of the appended claims.

FIG. 1 is a diagram illustrating a conventional variable gain amplifier.

FIG. 2A is a simplified diagram illustrating a communication systemaccording to embodiments of the present invention.

FIG. 2B is a simplified diagram illustrating an optical communicationsystem according to embodiments of the present invention.

FIG. 3A is a simplified diagram illustrating an NMOS-based differentialamplifier according to embodiments of the present invention.

FIG. 3B is a simplified diagram illustrating an NMOS-based VGA withvoltage supply according to embodiments of the present invention.

FIG. 4 provide circuit models of VGA circuits according to embodimentsof the present invention.

FIG. 5 provides a plot illustrating bandwidth of a VGA circuit accordingto embodiments of the present invention.

FIG. 6 provides a plot illustrating the relationship between bandwidthand gain of a VGA circuit according to embodiments of the presentinvention.

FIG. 7 provides a plot illustrating the relationship between frequencypeaking and gain of a VGA circuit according to embodiments of thepresent invention.

FIG. 8 is a simplified diagram illustrating a PMOS based VGA accordingto embodiments of the present invention.

FIG. 9 is a simplified diagram illustrating a BJT based VGA according toembodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed electrical circuits. According to aspecific embodiment, the present invention provides a variable gainamplifier that includes a first switch, which includes drain terminalcoupled to an inductor. A second switch is configured in parallel to theinductor, and the resistance value of the second switch is adjustable inresponse to a control signal. There are other embodiments as well.

As mentioned above, conventional VGAs are inadequate. FIG. 1 is adiagram illustrating a conventional VGA. The VGA in FIG. 1 is configuredas a source degeneration VGA that provides a good compromise among powerdissipation, linearity, and simplicity. To achieve a high gain withlarge gain control range, the degeneration device (switch M₃) istypically designed to be large. The parasitic capacitance at node X (orX′)—arising from the large dimension of M₃ and added to the capacitivecontribution of M_(B,1-2) and M_(1,2), thereby creating a zero in theoverall VGA transfer function—can be expressed as:

$\begin{matrix}{f_{z} = \frac{K( {V_{X} - V_{ctrl}} )}{2\pi C_{X}}} & {{{Equation}\mspace{20mu} 1}:}\end{matrix}$where, V_(X) is the DC voltage at X (or X′) reduced by the thresholdvoltage of M₃; C_(X) is the total capacitance at node X (or X′) and K isa constant. Equation 1 indicates that the zero frequency moves closer toorigin for lower VGA gain (i.e., V_(ctrl) is high), and may enter thesignal band of interest, thereby creating undesired peaking and/orbandwidth extension. The unintended peaking and bandwidth extension mayresult in sub-optimal utilization of the full-scale range (FSR) inanalog-to-digital converter (ADC) based receiver system due to the extratransient peaking, and they can cause unnecessary increase in noise orgroup-delay variation. Moreover, the unintended peaking and bandwidthextension create difficulty in equalization systems that prefer aconstant bandwidth as the gain changes.

There are some conventional solutions that address the variation ofbandwidth over gain range of the VGA, such as Gilbert VGA, which hasminimal bandwidth variation when implemented in CMOS technologies. AGilbert VGA uses the current steering gain control mechanism that doesnot vary the impedances in the signal path, unlike the topologyillustrated in FIG. 1. However, a Gilbert VGA, compared to the VGA inFIG. 1, is significantly more power hungry and may require highervoltage supply due to stacked devices. There are other conventional VGAdesigns, but unfortunately each with their own shortcomings.

The following description is presented to enable one of ordinary skillin the art to make and use the invention and to incorporate it in thecontext of particular applications. Various modifications, as well as avariety of uses in different applications will be readily apparent tothose skilled in the art, and the general principles defined herein maybe applied to a wide range of embodiments. Thus, the present inventionis not intended to be limited to the embodiments presented, but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

In the following detailed description, numerous specific details are setforth in order to provide a more thorough understanding of the presentinvention. However, it will be apparent to one skilled in the art thatthe present invention may be practiced without necessarily being limitedto these specific details. In other instances, well-known structures anddevices are shown in block diagram form, rather than in detail, in orderto avoid obscuring the present invention.

The reader's attention is directed to all papers and documents which arefiled concurrently with this specification and which are open to publicinspection with this specification, and the contents of all such papersand documents are incorporated herein by reference. All the featuresdisclosed in this specification, (including any accompanying claims,abstract, and drawings) may be replaced by alternative features servingthe same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

Furthermore, any element in a claim that does not explicitly state“means for” performing a specified function, or “step for” performing aspecific function, is not to be interpreted as a “means” or “step”clause as specified in 35 U.S.C. Section 112, Paragraph 6. Inparticular, the use of “step of” or “act of” in the Claims herein is notintended to invoke the provisions of 35 U.S.C. 112, Paragraph 6.

Please note, if used, the labels left, right, front, back, top, bottom,forward, reverse, clockwise and counter clockwise have been used forconvenience purposes only and are not intended to imply any particularfixed direction. Instead, they are used to reflect relative locationsand/or directions between various portions of an object.

FIG. 2A is a simplified diagram illustrating a communication system 200according to embodiments of the present invention. This diagram ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. Communication system 200includes a transmitter 210 and a receiver 220. The data from transmitter210 to receiver 220 is transmitted via the communication channel asshown. For example, the communication channel comprises electrical wiresconfigured to allow electrical signals to travel from transmitter 210 toreceiver 220. It is to be understood that transmitter 210 and receiver220 are used here to provide an example, and they may be implemented asvarious types of network entities. For example, transmitter 210 andreceiver 220 are both network entities that are capable of transmittingand receiving data, and they transceivers, data source,serializer/deserializer, and/or other network entities. Receiver 220includes VGA 221, implemented according to embodiments of the presentinvention, that adjusts the gain of incoming electrical signals. Thegain level of VGA 221 is based a control signal generated by DAC Control223. The amplified signal from VGA 221 is digitized by analog-to-digitalconverter 222 for processing in digital domain. VGA 221 receives fromcontroller 223 a control signal that adjusts the gain level. Receiver220 includes other components as well, such as timing recovery circuit,digital signal processing circuit, equalizer circuits, and/or others.Since VGA 221 is one of the first circuits that process the receivedsignal, it is desirable for output of VGA 221 to be as clean aspossible, which requires, among other things, a high level of linearityover a large operating bandwidth. It is to be appreciated that VGA 221,as implemented according to embodiments of the present invention, canprovide substantially constant bandwidth and highly linear output.

It is to be appreciated that VGAs have are widely implemented in variousimplementation of transceiver (TX and RX) systems, such as the system200 (e.g., the VGA is used in the front end of an ADC based electricalreceiver). Due to the variations in the transmitted and received signal,the amplified output from the VGA may saturate the ADC input. If the ADCsenses that its full-scale-range (FSR) is being utilized consistently,it attempts to reduce the VGA gain via DAC control, forming an automaticgain-control (AGC) loop.

FIG. 2B is a simplified diagram illustrating an optical communicationsystem 250 according to embodiments of the present invention. Thisdiagram is merely an example, which should not unduly limit the scope ofthe claims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. Transmitter 260 isconfigured to transmit data, in optical format, through the optical(e.g., fiber) channel as shown to receiver 280. For example,pulse-amplification modulation (PAM) protocol is used for transmissionthrough optical channel. Transimpedance amplifier (TIA) 270 isconfigured as a part of receiver 280, which converts optical signal toelectrical signal. The VGA of receiver 280 is configured to adjust gainof the electrical signal generated by TIA 270. The ADC digitizes theoutput of VGA. As explained above, it is important for the VGA toprovide substantially linear output over a large operating bandwidth. Invarious embodiments, the VGA gain is adjusted by the output of the DACcontrol as shown. The VGA in FIG. 2B is configured with a bandwidthextension mechanism as described below.

FIG. 3A is a simplified diagram illustrating an NMOS-based VGA 300according to embodiments of the present invention. This diagram ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. VGA 300 is configured as adifferential amplifier with differential inputs and differentialoutputs. Switches M₁ and M₂ are input switches for receivingdifferential input signals. In FIG. 3A, switches M₁ and M₂ are bothimplemented with NMOS transistors, and VGA 300 is referred to as NMOSbased differential amplifier. As described below, input switches can beimplemented with PMOS, BJT, or other types of switches.

The gain of VGA 300 can be adjusted by changing gate voltage of switchM₃. Configured between nodes X and X′ (respectively source terminals ofswitches M₁ and M₂), switch M₃ functions as a variable resistor. SignalV_(gain) effectively acts as the gain control signal that adjusts thegain of VGA 300 by changing effective resistance of switch M₃. As anexample, DAC-1, implemented as a part of control module 301, generatessignal V_(gain) based on the received DAC control code. For example,control module 301 uses DAC control code, an offset control signal, anda slope control signal.

Bias switches M_(B1) and M_(B2), both NMOS switches, are respectivelycoupled to source terminals of M₁ and M₂ and a supply voltage. The gateterminals of switches M_(B1) and M_(B2) are coupled to bias voltage Vb.

Differential input signals V_(in) are coupled to gate terminals ofswitches M₁ and M₂. The differential outputs V_(out) are coupled todrain terminals of switches M₁ and M₂. Inductors L₁ (one on each side)and resistors R_(D) (also one on each side) are respectively coupled tothe drain terminals of input switches M₁ and M₂. Switches M_(s) as shownare configured in parallel with the L₁ inductors (which are configuredas bandwidth defining inductors) as shown, and their gate terminals arecoupled to signal V_(BW). Switches M_(s) provide bandwidth extension forVGA 300. Switches M_(s) effectively function as variable resistor whoseresistance values are adjust by signal V_(BW). As an example, DAC-2generates signal V_(gain), which is associated with an offset signal anda slope control signal. The impedance value of switches M_(s) is controlvoltage dependent. For example, as impedance value of M_(s) switchesapproaches infinity, L₁ inductors is close to 100% effective; asimpedance value of M_(s) switches approaches zero, L₁ inductors aresubstantially bypassed. By adjusting L₁ effectiveness, M_(s) switchesimplements a mechanism to control bandwidth for VGA 300.

FIG. 3B is a simplified diagram illustrating an NMOS-based VGA 350 withvoltage supply according to embodiments of the present invention. Thisdiagram is merely an example, which should not unduly limit the scope ofthe claims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. VGA 350 is characterized bya similar architecture as VGA 300, but it has additional components anddifferent configurations. A cross-coupled device combination (M_(X)) 351is used to cancel a part of the Miller capacitance of the VGA 350 (e.g.,present at V_(in)). This ensures a constant capacitive load seen by thesource driving the VGA 350 irrespective of its gain, resulting ingain-independent BW at the input side of the VGA 350.

VGA 350 includes bias circuit 352 as shown. The biasing network, whichincludes M_(B1-5) and R_(B), is used to replicate the main VGA 350,albeit at a lower scale to reduce power dissipation. More specifically,M_(B3) is used to bias M_(B,1-2), while M_(B,4-5) along with R_(B) areused to establish a common mode voltage (V_(in,cm)) for the inputdifferential signal, V_(in) so that a fixed biasing current can beestablished through M_(1,2).

The gain of VGA 350 depends on the impedance of switches M_(Gain1) andM_(Gain2). Signal V_(control) <2> provides gain control voltage forswitch M_(Gain2). Signal V_(control) <1> functions as both gain controlvoltage (i.e., coupled to switch M_(gain)) and as bandwidth extensionvoltage (i.e., coupled to gates of M_(BW) switches). It is to be notedthat V_(control) <1> and V_(control) <2> are used to control M_(Gain1)and M_(Gain2) respectively, which means that the lower gain controlbranch M_(Gain2) can be designed separately from the higher gain controlbranch M_(Gain1). M Instead of the weak gain control branch M_(Gain2),the stronger gain control branch M_(Gain1) is used in conjunction withthe proposed bandwidth tuning technique (e.g., using switch M_(BW)). Invarious implementations, M_(Gain1) provide a bigger portion of the gaincontrol range that affects bandwidth, as compared to that of M_(Gain2).This allows to decouple linearity from VGA gain-range, and each can beoptimized separately. L₁ inductors are configured in series with theirrespective L₂ inductors. Gate terminals of common mode feedback switchesM_(N) are coupled to signal V_(cmfb), and the drain terminals of M_(N)switches are coupled between L₁ inductors and R_(D) resistors. Theresistor and NMOS combination (R_(D) and M_(N)) is configured at theload side of the VGA 350. The resistor R_(D) is needed to establish theoutput biasing voltage as well as the DC gain of the VGA. The parallelNMOS device (M_(N)) is used to both reduce voltage drop across R_(D),(which increases V_(GD) for M_(1,2) and thereby, linearity) and serve aspart of the common-mode feedback (CMFB) network.

The resistor and PMOS biasing combination (i.e., R_(S) and M_(B,1-2)),configured with bias circuit 352, is to reduce noise as well as toincrease voltage headroom for the input differential pairs (M_(1,2)).The increase of voltage headroom (by using M_(B,1-2)) results in highergate-to-drain voltage (V_(GD)) for M_(1,2). This greatly improves VGAlinearity by reducing output impedance modulation of M_(1,2) w.r.t. theoutput swing. Additionally, using resistance (R_(S)) allows M_(B,1-2) tobe smaller, thereby reducing their noise contribution.

To achieve substantially constant bandwidth at different gain controlsettings, a continually tunable resistance (M_(BW)) is configured inparallel to the bandwidth defining inductor, L₂, as shown in FIG. 3B.The use of a resistor in parallel to an inductor reduces theeffectiveness of inductor in presenting a higher impedance to higherfrequency components, thereby decreasing the bandwidth. For example, theresistance value of switch M_(BW) is R_(s). Quantitatively, theimpedance of resistor (R_(S)) and inductor (L₂) configured in parallelcan be expressed in Equation 2 below:

$\begin{matrix}{{{Equation}\mspace{25mu} 2}:} & \; \\{{Z_{L2}( {j\;\omega}\; )} = {\frac{{R_{S}( {\omega L_{2}} )}^{2}}{R_{S^{2}} + ( {\omega L_{2}} )^{2}} + {j\frac{R_{S}^{2}\omega L_{2}}{R_{S^{2}} + ( {\omega L_{2}} )^{2}}}}} & (2)\end{matrix}$

Equation 2 indicates that for large values of R_(S) (R_(S)>>ωL₂), theimpedance is reduced to Z_(L2)(jω)≈jωL₂, indicating that the inductanceis not affected by R_(S). On the other hand, for smaller value of R_(S),the inductance vanishes since Z_(L2)(jω)≈R_(S) for R_(S)<<ωL₂.Therefore, if the bandwidth is a strong function of L₂, then tuningR_(S) will tune the bandwidth.

FIG. 4 provide circuit models of VGA circuits according to embodimentsof the present invention. This diagram is merely an example, whichshould not unduly limit the scope of the claims. One of ordinary skillin the art would recognize many variations, alternatives, andmodifications. For example, a simplified model of the VGA 350 is shownin FIG. 4(a), which encapsulates the variation of the zero frequencyw.r.t. control voltage (e.g., described in Equation 1) inside thefrequency dependent current source model of M_(1,2) (I_(in)). For largeR_(S), the amplifier model is reduced to that of a fixed gain amplifier,as shown in FIG. 4(b). For an uneven output capacitor loading at thenear and far end of the VGA (i.e., capacitors C₁ and C₂, where C₁ is thenear end loading arising at the drain nodes of input switches M_(1,2)and C₂ is the capacitance from the next stage), it has been shown thatthe specific configuration of L₁ & L₂ in FIG. 4(b) yields the highestbandwidth when C₂>C₁. For example, for a split ratio of α=0.25 andR_(S)→∞, it can be shown that the maximum achievable bandwidth of theproposed architecture is

${{BW_{Max}} \approx \frac{4}{2\pi R_{D}C}},$where C═C₁+C₂ and R_(D) is the drain resistance (to put the result intoperspective, the bandwidth would only be 1/2πR_(D)C if no inductor wasused). Here it is assumed that the DC impedance (R_(D)) is maintained atthe first resonance frequency of the π-network

$( \frac{1}{2\pi\sqrt{L_{2}C_{1}}} ),$which, in turn is assumed to be equal to the resonance frequency of ashunt peaked amplifier—as shown FIG. 4(c)—is used for specificimplementations. The values for L₁ & L₂ can then be derived from thesetwo boundary conditions for a given loading condition (R_(D), C₁ andC₂).

A practical implementation of R_(S) is using a linear mode NMOS (e.g.,switch M_(BW) as shown in FIGS. 3A and 3B), which yields an inverserelationship between V_(ctrl) and bandwidth, meaning that the dominantpole will decrease as V_(ctrl) increases. As a result, this pole can beused to cancel the zero arising from C_(X) (e.g., see FIG. 1) to yieldsubstantially constant overall BW, since both the pole and zerodecreases with increasing control voltage (e.g., see Equation 1).

It is to be appreciated that there are implementation challenges arisebeyond the simplistic assumptions stated above. First, due to thecomplicated relationship between the bandwidth and V_(ctrl), thebandwidth control curve of Z_(out)—of model illustrated in FIG. 4(a)—ishighly non-linear as shown in the simulation results in FIG. 5. FIG. 5provides a plot illustrating bandwidth of a VGA circuit according toembodiments of the present invention. It is possible to counteract thenon-linearity by inserting an inverse function to the non-linearrelation between V_(ctrl) and M_(BW), which can be implemented using aDAC, or other means. However, the compensated BW variations in variousimplementations (e.g., as illustrated in FIG. 3A and FIG. 3B) arisingfrom the non-linear control curve is small (e.g., as illustrated in FIG.6), and may be acceptable for most applications. FIG. 6 provides a plotillustrating the relationship between bandwidth and gain of a VGAcircuit according to embodiments of the present invention.

Anther implementation challenge arises if the simplistic controlarchitecture—as illustrated in the FIG. 3B example—is adopted (where thesame control voltage, V_(ctrl) is used to control both gain andbandwidth). It is needed to center the control curves for both pole(e.g., see FIG. 5) and the parasitic zero (e.g., see Equation 1) so thata linear approximation for pole-zero cancellation in the region ofinterest is achieved (similar to the dotted line in FIG. 5). In variousembodiment, the present invention provides a technique of aligning thecenter that uses different threshold voltage device for NMOS (M_(BW))and PMOS (M_(Gain1)). For certain implementation, a Monte-Carlosimulation on the architecture reveals that the mismatch arising due towith choice is suitable for certain implementations. It is worthmentioning that the slope of the control curve can be adjusted to matchEquation 1 by properly sizing the NMOS device (M_(BW)) with respect tothe PMOS device (M_(Gain1)).

The results of the proposed technique are illustrated in FIGS. 6 and 7,indicating that the proposed technique reduces the BW variation by about12% and suppresses peaking from 5 dB to <2 dB. FIG. 7 provides a plotillustrating the relationship between frequency peaking and gain of aVGA circuit according to embodiments of the present invention. It isalso worth mentioning that the application of the proposed inductortuning technique is not just limited to the above architectures (e.g.,VGA 300 and VGA 350), but can also be extended to the commonly usedtriple-resonance network (TRN) and shunt peaking VGA architectures andtheir NMOS counterparts.

FIG. 8 is a simplified diagram illustrating a PMOS based VGA accordingto embodiments of the present invention. This diagram is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. As an example, input switches M₁ and M₂ areimplemented with PMOS switches, and the differential amplifier isconfigured accordingly. M_(S) switches are configured in parallelrelative the L₁ inductors as shown. A control module—with DAC control,offset control, and slope control—provides control signal V_(BW) andcontrol signal V_(Gain). V_(BW) adjusts impedance value of switchesM_(S), which affects the effectiveness of L₁ inductors. V_(Gain) adjuststhe impedance value of switch M₃, which affects the gain of the VGA. VGAin FIG. 8 also includes drain resistors R_(D) and bias switches M_(B1)and M_(B2).

FIG. 9 is a simplified diagram illustrating a BJT based VGA according toembodiments of the present invention. This diagram is merely an example,which should not unduly limit the scope of the claims. One of ordinaryskill in the art would recognize many variations, alternatives, andmodifications. As an example, input switches M₁ and M₂ are implementedwith bipolar junction transistor (BJT) switches, and the differentialamplifier is configured accordingly. M_(S) switches are configured inparallel relative the L₁ inductors as shown. A control module—with DACcontrol, offset control, and slope control—provides control signalV_(BW) and control signal V_(Gain). V_(BW) adjusts impedance value ofswitches M_(S), which affects the effectiveness of L₁ inductors.V_(Gain) adjusts the impedance value of switch M₃, which affects thegain of the VGA. VGA in FIG. 8 also includes resistors R_(D) and biasswitches M_(B1) and M_(B2). It is to be appreciated that there arecertain advantages associated with BJT based (i.e., with BJT as inputswitches) VGAs. For example, BJT switches can offer higher speed andgain compared to CMOS switches, but BJT switches cannot be used toimplement a voltage-controlled resistor (i.e., used to implementedswitches M_(S) and M₃). In various embodiments, a VGA implemented BJTdevices and CMOS devices can be manufactured in available processes.

While the above is a full description of the specific embodiments,various modifications, alternative constructions and equivalents may beused. Therefore, the above description and illustrations should not betaken as limiting the scope of the present invention which is defined bythe appended claims.

What is claimed is:
 1. A variable gain amplifier comprising: a firstswitch comprising a first gate terminal and a first drain terminal, thefirst gate terminal being coupled to a first input voltage of adifferential input signal; a second switch comprising a second gateterminal and a second drain terminal, the second gate terminal beingcoupled to a second input voltage of the differential input signal; afirst output terminal coupled to the first drain terminal; a secondoutput terminal coupled to the second drain terminal; a first inductorcoupled to the first drain terminal; a second inductor coupled to thesecond drain terminal; a third switch comprising a third gate terminaland coupled in parallel with the first inductor; a fourth switchcomprising a fourth gate terminal and coupled in parallel with thesecond inductor, the third switch and the fourth switch receiving, froma control module, a bandwidth control signal that varies a bandwidth ofthe variable gain amplifier; and a fifth switch that receives, from thecontrol module, a gain control signal that varies a gain of the variablegain amplifier, the first switch and the second switch being configuredto output, via the first output terminal and the second output terminal,a differential output signal in accordance with (i) the gain controlsignal and (ii) the bandwidth control signal.
 2. The variable gainamplifier of claim 1, further comprising a resistor coupled to the firstinductor.
 3. The variable gain amplifier of claim 1, further comprisinga resistor coupled to the fifth switch.
 4. The variable gain amplifierof claim 1, further comprising a sixth switch coupled to the fifthswitch, the sixth switch comprising a sixth gate terminal coupled to abias signal.
 5. The variable gain amplifier of claim 1, wherein thefirst switch comprises an NMOS switch.
 6. The variable gain amplifier ofclaim 1, further comprising a cross-coupled device coupled to the firstswitch and the second switch.
 7. The variable gain amplifier of claim 1,wherein the third switch comprises a PMOS switch.
 8. A variable gainamplifier comprising: a first switch comprising a first base terminaland a first collector terminal, the first base terminal being coupled toa first input voltage; a second switch comprising a second base terminaland a second collector terminal, the second base terminal being coupledto a second input voltage; a first output terminal coupled to the firstcollector terminal; a second output terminal coupled to the secondcollector terminal; a first inductor coupled to the first collectorterminal; a second inductor coupled to the second collector terminal; athird switch comprising a first gate terminal and configured in parallelto the first inductor; a fourth switch comprising a second gate terminaland configured in parallel to the second inductor, the third switch andthe fourth switch configured to receive, from a control module, abandwidth control signal that varies a bandwidth of the variable gainamplifier; and a fifth switch configured to receive, from the controlmodule, a gain control signal that varies a gain of the variable gainamplifier, the first switch and the second switch being configured tooutput, via the first output terminal and the second output terminal, adifferential output signal in accordance with (i) the gain controlsignal and (ii) the bandwidth control signal.
 9. The variable gainamplifier of claim 8, wherein the first switch comprises a bipolarjunction transistor (BJT) switch.
 10. The variable gain amplifier ofclaim 9, wherein the BJT switch is NPN type.
 11. The variable gainamplifier of claim 9, wherein the third switch comprises a MOSFETtransistor.
 12. The variable gain amplifier of claim 8, wherein thefirst gate terminal is coupled to the gain control signal.
 13. Thevariable gain amplifier of claim 9, wherein the gain control signal ischaracterized by a slope value and an offset value.
 14. A receiverapparatus comprising: an input terminal for receiving incoming datasignals; a variable gain amplifier coupled to the input terminal, thevariable gain amplifier comprising a first switch and an inductor, thefirst switch being coupled to the input terminal, the first switchcomprising (i) a source terminal coupled to a variable resistor and (ii)a drain terminal coupled to the inductor, the variable resistor beingconfigured to receive, from a control module, a first control signalthat varies a gain of the variable gain amplifier, the variable gainamplifier further comprising a second switch coupled in parallel withthe inductor, the second switch being configured to receive, from thecontrol module, a second control signal that varies a bandwidth of thevariable gain amplifier; and an analog to digital converter (ADC)configured to generate a digital signal based on the incoming datasignal, the ADC being coupled to the drain terminal.
 15. The receiverapparatus of claim 14 further comprising a transimpedance amplifier forconverting incoming optical signals to the incoming data signal.
 16. Thereceiver apparatus of claim 14 wherein the variable gain amplifierfurther comprises the control module.